53 C# Interview Questions for Experienced Professionals PDF 2025

If you’re a C# developer with years of experience, preparing for an interview means going beyond basic concepts. Interviewers often expect you to demonstrate practical knowledge of real-world software design, architecture patterns, performance optimization, multithreading, and enterprise-level practices.

Here we have added Top C# Interview Questions that align with what hiring managers and senior tech leads typically ask. Including different levels of C# interview questions, Such as 3 years, 5 years, 7 years, and 10 years experience.

We will cover core concepts, advanced implementations, design patterns, and hands-on coding challenges. This guide is tailored to help you build solid, interview ready confidence.

C# Interview Questions for 3 Years Experience

Que 1. How do you implement dependency injection in C#?

Dependency Injection (DI) allows objects to receive their dependencies from external sources. In C#, it’s commonly implemented using constructors, method injection, or property injection. For example, with constructor injection:

public class OrderService
{
    private readonly IOrderRepository _orderRepository;
    public OrderService(IOrderRepository orderRepository)
    {
        _orderRepository = orderRepository;
    }
}

This helps with loose coupling and unit testing.

Que 2. What is the difference between ref and out parameters?

Both are used to pass arguments by reference, but ref requires the variable to be initialized before it’s passed, while out doesn’t. The out parameter must be assigned inside the called method.

Que 3. How does garbage collection work in .NET?

Garbage collection in .NET is automatic and manages memory by reclaiming unused objects. It works in generations (0, 1, 2), and periodically checks for objects no longer in use, freeing up memory without manual intervention.

Que 4. How would you handle exceptions in async methods?

You should use try-catch blocks inside async methods to catch exceptions. Also, unhandled exceptions in tasks can be caught using .ContinueWith or observing Task.Exception.

Que 5. What are extension methods in C#?

Extension methods allow you to “add” methods to existing types without modifying them. They must be declared in a static class and the first parameter should have the this modifier:

public static class StringExtensions
{
    public static bool IsNullOrEmpty(this string value)
    {
        return string.IsNullOrEmpty(value);
    }
}

Que 6. What is boxing and unboxing in C#?

Boxing converts a value type to an object type, and unboxing is the reverse. It introduces performance overhead due to heap allocation:

int x = 123;
object obj = x; // Boxing
int y = (int)obj; // Unboxing

Que 7. What is the difference between IEnumerable and IQueryable?

IEnumerable is used for in-memory collections and is evaluated immediately. IQueryable is used for querying databases and supports deferred execution, allowing LINQ to SQL.

Que 8. When would you use async and await?

Use async and await when performing non-blocking operations, such as I/O-bound tasks like API calls, file reading, or database queries, to keep the UI or threads responsive.

Que 9. What are value types and reference types in C#?

Value types (e.g., int, struct) store data directly and are stored in the stack. Reference types (e.g., class, object) store a reference to the data, which resides in the heap.

Que 10. How do you create a thread-safe class in C#?

Use synchronization techniques like lock, Mutex, or concurrent collections. For example:

private readonly object _lock = new object();
public void SafeMethod()
{
    lock (_lock)
    {
        // Thread-safe operations
    }
}

Que 11. What’s the use of the using statement in C#?

The using statement ensures that IDisposable objects like FileStream or DbConnection are automatically disposed after use, preventing resource leaks.

Que 12. Explain the differencebetween abstract class and interface.

• Abstract class can have method implementations and state (fields).
• Interface only defines contracts (methods, properties) and doesn’t allow implementation (until C# 8 default methods).

Interfaces allow multiple inheritance; abstract classes do not.

Que 13. How do you handle large datasets efficiently in C#?

Use lazy evaluation with yield return, asynchronous processing, paging techniques with Skip() and Take(), or streaming data in chunks to avoid memory overload.

Que 14. What is a delegate and how do you use it?

A delegate is a type that represents references to methods with a specific signature. It’s used for implementing callbacks and event handlers.

public delegate void Notify();
Notify del = ShowMessage;

Que 15. How do you optimize LINQ queries for performance?

• Use ToList() only when needed.
• Avoid multiple enumerations.
• Use Select and Where smartly.
• For large datasets, prefer IQueryable over IEnumerable.

C# Interview Questions for 5 Years Experience

Que 16. What is the difference between Task and Thread in C#?

A Thread is a low-level representation of a separate execution path, while a Task is a higher-level abstraction built on top of threads and managed by the Task Parallel Library (TPL). Tasks are lightweight, easier to manage, and support features like continuation, cancellation, and exception handling.

// Example using Task
Task.Run(() => DoWork());

// Example using Thread
Thread t = new Thread(DoWork);
t.Start();

Que 17. How do you implement caching in a C# application?

You can implement caching using the built-in MemoryCache class for in-memory caching or use distributed solutions like Redis for larger applications.

var cache = MemoryCache.Default;
cache.Add("key", data, DateTimeOffset.Now.AddMinutes(10));

Benefits:

• Reduces database load
• Improves performance
• Supports sliding and absolute expiration

Que 18. What are SOLID principles in C#?

SOLID is a set of five principles for writing maintainable and scalable code:

• S – Single Responsibility Principle
• O – Open/Closed Principle
• L – Liskov Substitution Principle
• I – Interface Segregation Principle
• D – Dependency Inversion Principle

These principles are fundamental for clean architecture and testable applications.

Que 19. What is boxing and unboxing in C#?

Boxing is the conversion of a value type to a reference type (object). Unboxing is the reverse.

int x = 42;
object obj = x;        // Boxing
int y = (int)obj;      // Unboxing

Boxing causes performance overhead due to heap allocation, so avoid it in performance-critical paths.

Que 20. How is dependency injection implemented in .NET Core?

Dependency Injection (DI) is implemented using the built-in IServiceCollection in Startup.cs.

// Registering service
services.AddScoped<IMyService, MyService>();

// Consuming in controller
public MyController(IMyService service) {
    _service = service;
}

Benefits:

• Improves testability
• Promotes loose coupling
• Simplifies service management

Que 21. How do you handle exceptions in asynchronous methods?

Use try-catch blocks inside async methods. Exceptions in tasks must be awaited or explicitly observed using .Exception.

try {
    await DoAsyncOperation();
} catch (Exception ex) {
    // Handle error
}

Avoid mixing .Result or .Wait() with async code to prevent deadlocks.

Que 22. What is reflection in C# and when is it used?

Reflection lets you inspect and interact with metadata at runtime. It’s commonly used in frameworks, serializers, and plugin loaders.

Type type = typeof(Person);
PropertyInfo prop = type.GetProperty("Name");
object value = prop.GetValue(obj);

Use it cautiously since it’s slower and bypasses compile-time checks.

Que 23. What is a CancellationToken and how is it used?

CancellationToken enables cooperative cancellation in async tasks. You check IsCancellationRequested or register callbacks to stop work when cancellation is triggered.

if (token.IsCancellationRequested) {
    return;
}

You typically pass it to long-running operations to allow graceful exits.

Que 24. What are extension methods in C#?

Extension methods allow you to “add” new methods to existing types without modifying them.

public static class MyExtensions {
    public static bool IsNullOrEmpty(this string str) {
        return string.IsNullOrEmpty(str);
    }
}

// Usage
"hello".IsNullOrEmpty();

They’re especially useful in LINQ and utility classes.

Que 25. How do lock and Monitor differ in C#?

Here is a tabular comparison between lock and Monitor:

Feature	lock	Monitor
Syntax	Simple (lock(obj) {})	More verbose (Enter/Exit)
Reentrancy	Not reentrant	Not reentrant
Wait/Pulse	Not supported	Supported
Performance	Slightly slower for complex logic	More flexible

Use lock for simple synchronization, and Monitor for advanced signaling (Pulse, Wait, etc.).

Que 26. What is the difference between abstract class and interface?

• Abstract class can have implementations; interface cannot (prior to C# 8).
• A class can inherit only one abstract class but multiple interfaces.
• Interfaces are used for defining capabilities, while abstract classes define a base behavior.

interface IWorker {
    void Work();
}

abstract class Employee {
    public abstract void GetSalary();
}

Que 27. How do you use tuples in C#?

Tuples allow returning multiple values from a method.

(string Name, int Age) GetPerson() {
    return ("John", 30);
}

// Usage
var person = GetPerson();
Console.WriteLine(person.Name);

They’re a lightweight alternative to custom classes for temporary data grouping.

Que 28. How do you manage versioning in .NET assemblies?

Use assembly attributes in AssemblyInfo.cs:

[assembly: AssemblyVersion("1.0.0.0")]
[assembly: AssemblyFileVersion("1.0.0.0")]

Semantic versioning helps you communicate backward-incompatible changes and feature additions clearly.

Que 29. What are value types and reference types in C#?

• Value Types (int, struct): Stored in stack, copied by value.
• Reference Types (class, array): Stored in heap, copied by reference.

Understanding this distinction is important for memory management and performance optimization.

Que 30. What is the difference between IEnumerable and IQueryable?

• IEnumerable executes queries in-memory and supports LINQ to Objects.
• IQueryable executes queries at the data source (e.g., SQL Server) and supports LINQ to Entities.

Use IQueryable for deferred execution and filtering at the database level.

Also Check: Software Engineer Interview Questions and Answers

C# Interview Questions for 7 Years Experience

Que 31. How do you implement custom middleware in ASP.NET Core?

To implement custom middleware, you create a class with an Invoke or InvokeAsync method and register it in the pipeline.

public class LoggingMiddleware {
    private readonly RequestDelegate _next;
    
    public LoggingMiddleware(RequestDelegate next) {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context) {
        Console.WriteLine("Request path: " + context.Request.Path);
        await _next(context);
    }
}

// In Startup.cs
app.UseMiddleware<LoggingMiddleware>();

Middlewares are powerful for request logging, exception handling, and authorization.

Que 32. What are the differences between struct and class in C#?

Feature	struct	class
Type	Value type	Reference type
Inheritance	Cannot inherit from another struct or class	Can inherit from another class
Default constructor	Not allowed	Allowed
Memory	Stack	Heap
Performance	Faster for small data	Slower, GC overhead

Use structs for small, immutable data models.

Que 33. How would you ensure thread safety in a multi-threaded C# application?

Use thread-safe constructs:

• lock statements for critical sections
• ConcurrentDictionary, BlockingCollection for collections
• Interlocked class for atomic operations
• SemaphoreSlim for resource control

Example using lock:

private readonly object _sync = new object();

lock (_sync) {
    sharedResource++;
}

Thread safety is essential for avoiding race conditions and deadlocks.

Que 34. What is method hiding and how does it differ from overriding?

Method hiding uses the new keyword, while overriding uses override.

class Base {
    public void Show() => Console.WriteLine("Base");
}

class Derived : Base {
    public new void Show() => Console.WriteLine("Derived");
}

Overridden methods use runtime polymorphism; hidden methods are resolved at compile time.

Que 35. How do you prevent memory leaks in .NET applications?

• Dispose unmanaged resources with IDisposable and using
• Avoid static event handlers without unsubscribing
• Use WeakReference if appropriate
• Monitor memory using tools like dotMemory or Visual Studio diagnostics

using (var stream = new FileStream("file.txt", FileMode.Open)) {
    // auto-disposed
}

Memory leaks in managed code often come from event subscriptions and long-lived object graphs.

Que 36. How do async and await improve performance in C# applications?

They allow non-blocking code execution, releasing the thread to do other work while waiting for I/O-bound tasks.

public async Task<string> GetDataAsync() {
    var response = await httpClient.GetStringAsync(url);
    return response;
}

It’s ideal for I/O-bound operations like file access, network requests, or database calls.

Que 37. What’s the difference between AutoMapper and manual mapping?

• AutoMapper: Automatically maps similar properties between objects
• Manual Mapping: Gives full control but is verbose

Use AutoMapper for clean and fast development, and manual mapping when performance and flexibility are critical.

CreateMap<User, UserDto>();
var dto = mapper.Map<UserDto>(user);

Que 38. How do you enforce encapsulation and immutability in C#?

• Use private setters
• Use read-only fields
• Mark class as sealed if not inheritable

public class Customer {
    public string Name { get; }
    public Customer(string name) {
        Name = name;
    }
}

Encapsulation keeps the internal state hidden, and immutability improves thread safety.

Que 39. How do you implement global exception handling in .NET Core?

Use app.UseExceptionHandler() in Startup.cs and optionally a custom middleware.

app.UseExceptionHandler("/Home/Error");

Or with middleware:

app.Use(async (context, next) => {
    try {
        await next();
    } catch (Exception ex) {
        // Log and respond
    }
});

Ensures consistent error responses and avoids leaking stack traces.

Que 40. What is the difference between virtual, override, and new keywords in C#?

Keyword	Purpose
virtual	Allows a base class method to be overridden
override	Provides new implementation of a virtual method
new	Hides a member from the base class

Use override to extend behavior and new when you need entirely separate logic.

C# Interview Questions for 10 Years Experience

Que 41. How do you design a scalable and loosely coupled C# application?

Design principles and patterns to use:

• Apply SOLID principles (especially Dependency Inversion)
• Use interfaces and Dependency Injection
• Break functionality into layers: Presentation, Business, Data
• Use event-driven or CQRS if required
• Opt for clean architecture or hexagonal architecture

Example using DI:

public interface IEmailService {
    void Send(string to, string subject);
}

public class EmailService : IEmailService {
    public void Send(string to, string subject) {
        // send logic
    }
}

In Program.cs:

services.AddScoped<IEmailService, EmailService>();

Que 42. What strategies would you use to improve application performance in C#?

• Optimize LINQ queries
• Reduce boxing/unboxing
• Use async/await properly
• Apply caching (e.g., MemoryCache)
• Profile with dotTrace, PerfView, or Visual Studio Profiler
• Use Span or Memory<T> for memory-efficient operations

Que 43. What is the role of Span in performance optimization?

Span<T> allows you to work with slices of arrays, strings, or memory regions without allocations.

Span<char> span = stackalloc char[100];
// No heap allocation, very fast

Ideal for high-performance applications, parsing, and memory-critical operations.

Que 44. How would you handle versioning in Web APIs built using C#?

Approaches to API versioning:

• URL based: /api/v1/products
• Query string: /api/products?v=1
• Header-based: Accept: application/vnd.myapi.v1+json

Use Microsoft.AspNetCore.Mvc.Versioning:

services.AddApiVersioning(options => {
    options.AssumeDefaultVersionWhenUnspecified = true;
    options.DefaultApiVersion = new ApiVersion(1, 0);
});

Que 45. How do you implement Domain-Driven Design (DDD) in C# projects?

• Use Value Objects, Entities, Aggregates
• Keep domain logic within the domain layer
• Use repositories to abstract persistence
• Avoid anemic models — encapsulate behavior

Example of a value object:

public class Money {
    public decimal Amount { get; }
    public string Currency { get; }

    public Money(decimal amount, string currency) {
        Amount = amount;
        Currency = currency;
    }
}

Que 46. What is the difference between Task.WhenAll and Task.WaitAll?

Method	Description
Task.WhenAll	Asynchronous, non-blocking, returns Task
Task.WaitAll	Blocking, waits for all tasks to finish synchronously

Prefer Task.WhenAll in async methods to avoid blocking the thread.

await Task.WhenAll(task1, task2);

Que 47. How do you handle distributed transactions in microservices built with C#?

Options include:

• Outbox pattern
• Eventual consistency using messaging (RabbitMQ, Kafka)
• Use Saga pattern or Orchestration approach
• Avoid distributed transactions unless necessary

Tools: MassTransit, CAP, NServiceBus, Dapr, etc.

Que 48. How do you secure sensitive configuration values in .NET applications?

• Use User Secrets for development
• Use Azure Key Vault, AWS Secrets Manager, or HashiCorp Vault for production
• Use IConfiguration to inject config securely

builder.Configuration.AddUserSecrets<Startup>();

Que 49. How would you implement retry logic in C#?

Use libraries like Polly:

var policy = Policy
    .Handle<HttpRequestException>()
    .WaitAndRetry(3, retryAttempt => TimeSpan.FromSeconds(2));

policy.Execute(() => CallExternalService());

Helps in handling transient faults like network failures or timeouts.

Que 50. What are the differences between Service Locator and Dependency Injection?

Feature	Service Locator	Dependency Injection
Coupling	High	Low
Readability	Poor (hidden dependencies)	Better (explicit dependencies)
Testability	Harder	Easier

Avoid Service Locator anti-pattern; prefer constructor injection for better design.

Que 51. How do you implement logging in enterprise-level C# applications?

Use structured logging and logging abstractions such as:

• ILogger from Microsoft.Extensions.Logging
• Use providers: Serilog, NLog, or log4net
• Configure log levels (Information, Warning, Error)
• Write logs to multiple sinks (File, Console, DB, Cloud)

Example using Serilog:

Log.Logger = new LoggerConfiguration()
    .WriteTo.File("logs/app.log")
    .CreateLogger();

Log.Information("Application started");

Always include correlation IDs and context for tracing.

Que 52. Explain the Repository and Unit of Work patterns in C#.

• Repository Pattern abstracts data access logic.
• Unit of Work ensures a single transaction for multiple operations.

Example:

public interface ICustomerRepository {
    Customer GetById(int id);
    void Add(Customer customer);
}

public interface IUnitOfWork {
    ICustomerRepository Customers { get; }
    void Commit();
}

Used together to maintain transactional consistency and separation of concerns.

Que 53. How do you apply Clean Architecture in large-scale C# systems?

Clean Architecture layers:

• Entities: Core business rules
• Use Cases / Application Layer: Business logic orchestration
• Interface Adapters: Controllers, Presenters, Views
• Frameworks & Drivers: UI, DB, external tools

Key ideas:

• Dependency flows inward
• Use interfaces for boundaries
• Enforce separation of concerns
• Use Dependency Injection for plug-in architecture

Clean architecture improves testability, maintainability, and scalability of complex systems.

Also Check: Python Interview Questions and Answers

C# Interview Questions for Experienced PDF

We have also prepared a PDF that includes all the C# interview questions and answers for experienced shared in this guide. This PDF is ideal for offline practice, last minute revision, or sharing with your peers and colleagues

FAQs: C# Interview Questions for Experienced